1. Field of the Invention
The invention relates to a catalyst overheat prevention apparatus.
2. Description of the Related Art
A catalyst for purifying exhaust gas is provided in an internal combustion engine (will hereinafter be referred to as “engine”). The purification capacity of such a catalyst may decrease when the catalyst is overheated by, for example, high-temperature exhaust gas. More specifically, such a decrease in the purification capacity of a catalyst may be caused by, for example, the temperature of the catalyst increasing beyond its activation temperature. In order to maintain a desired purification capacity of a catalyst, control is executed in which the fuel injection amount is increased so that the exhaust gas temperature decreases due to the vaporization heat of fuel and thus the catalyst is cooled down. Such fuel injection amount increase control will hereinafter be referred to as “OT (Over-Temperature Protection) fuel increase control”, and the amount by which the fuel injection amount is increased in OT fuel increase control will hereinafter be referred to as “OT fuel increment”. It is to be noted that OT fuel increase control is an example of control for preventing overheating of a catalyst (catalyst overheat prevention control).
Japanese Patent Application Publication No. 2003-343242 describes a technique of estimating the temperature of a catalyst in accordance with the engine speed and the engine load and then determining, based on the estimated temperature of the catalyst, whether the catalyst is presently overheated (will hereinafter be referred to as “OT determination”).
However, when OT fuel increase control is being executed, the fuel injection amount may be increased excessively, resulting in an increase in the exhaust emissions, such as CO (carbon oxide) and HC (hydrocarbon) and a reduction of the fuel economy, as will hereinafter be described in more detail with reference to FIGS. 9A and 9B. FIG. 9A illustrates, by way of example, a relation between time and the temperature of a catalyst, and FIG. 9B illustrates, by way of example, a relation between time and the OT fuel increment. In FIGS. 9A and 9B, the horizontal axis represents time. The vertical axis in FIG. 9A represents the catalyst temperature, and the vertical axis in FIG. 9B represents the OT fuel increment. In FIG. 9A, the broken line represents a convergence temperature T1 of the catalyst, the dotted line represents a present temperature T2 of the catalyst, and the solid line represents an actual temperature T4 of the catalyst. In FIG. 9B, the broken line represents an OT fuel increase control base value D1 and the solid line represents an OT fuel increase control correction factoring-in value D2. FIGS. 9A and 9B illustrate a state where an excess air ratio λ is 1, that is, a state where the engine is running at the stoichiometric air-fuel ratio.
The OT fuel increase control can be executed when the convergence temperature T1 of the catalyst and the present temperature T2 of the catalyst are equal to or higher than an OT determination temperature T3. Note that “convergence temperature” is the temperature on which the temperature of the exhaust system converges when the engine is running with a given amount of intake air and at a given speed, and “present temperature” is the temperature of the catalyst that is determined through, for example, moderating based on the convergence temperature. Further, note that “OT determination temperature” is a reference temperature used in determining whether the catalyst is presently overheated. That is, when the present temperature is equal to or higher than the OT determination temperature, it is determined that the catalyst is presently overheated. With regard to the determination as to whether to execute the OT fuel increase control (will hereinafter be referred to as “OT fuel increase control execution determination”), the OT determination temperature T3 has a hysteretic characteristic with respect to the convergence temperature T1. That is, when the convergence temperature T1 and the OT determination temperature T3 are compared with each other in the OT fuel increase control execution determination, the OT determination temperature T3 is used if the vehicle is accelerating, and a hysteretic OT determination temperature T3′ that has a hysteretic characteristic and is lower than the OT determination temperature T3 is used if the vehicle is decelerating.
The convergence temperature T1 and the present temperature T2 start increasing at time t1 in response to the start of acceleration of the vehicle. Then, at time t2, a situation occurs where the convergence temperature T1 and the present temperature T2 are both equal to or higher than the OT determination temperature T3, and therefore the OT fuel increase control is started. For example, the OT fuel increase control is executed such that the temperature of the catalyst decreases down to the OT determination temperature T3. Thus, the actual temperature T4 of the catalyst is reduced to the OT determination temperature T3. Then, at time t3, the convergence temperature T1 starts decreasing in response to the start of deceleration of the vehicle. At this time, the actual temperature T4 also starts decreasing and then becomes lower than the OT determination temperature T3. As such, normally, the OT fuel increase control is finished at time t3.
However, the OT fuel increase control is finished in response to the present temperature T2 becoming lower than the OT determination temperature T3 or in response to the convergence temperature T1 becoming lower than the hysteretic OT determination temperature T3′. Thus, the OT fuel increase control is finished at time t5. That is, the time at which the OT fuel increase control is finished is delayed from time t3 to time t5. This delay occurs due to the deviation of the present temperature T2 from the actual temperature T4 at time t3 at which the vehicle starts decelerating, which deviation has been caused as a result of execution of the OT fuel increase control. Further, due to such a deviation of the present temperature T2 from the actual temperature T4, the time at which the OT fuel increase control is started is advanced from time t7 to time t6 when the vehicle is accelerating again after accelerating and decelerating repeatedly, and also the time at which the OT fuel increase control is thereafter finished is delayed from time t8 to time t10 when the vehicle is decelerating again. As such, surplus fuel of an amount corresponding to the hatched regions shown in FIG. 9B is consumed at the time of executing the OT fuel increase control.